Many birds also have UV patterns in their feathers. In 1998, two independent teams realized that much of the “blue” plumage of blue tits actually reflects a lot of UV; as one of them wrote, “Blue tits are ultraviolet tits.” To humans, these birds all look much the same. But thanks to their UV patterns, males and females look very different from each other. The same is true for more than 90 percent of songbirds whose sexes are indistinguishable to us, including barn swallows and mockingbirds.
